Field of the Invention
The present invention relates to an axial-flow turbine, and more particularly to a turbine nozzle and a moving blade forming a fluid passage of the axial-flow turbine.
A variety of techniques relating to the axial-flow turbine have been employed to improve an internal efficiency of the turbine so as to improve the performance of the same. Since a secondary flow loss among internal losses experienced with the axial-flow turbine is a loss of a type common to all stages of the turbine, a contrivance that is capable of preventing the above-mentioned loss has been required.
FIG. 19 shows cross sections of turbine stages of a usual axial-flow turbine including nozzle blades and moving blades. Referring to FIG. 19, a plurality of nozzle blades 4 are radially secured between an outer diaphragm ring 2 and an inner diaphragm ring 3 which are fit to a turbine casing 1 so that a nozzle blade passage is formed. A plurality of moving blades 6 is disposed at a downstream side of the nozzle blade passage. Each of moving blades 6 is sequentially implanted in the outer surface of the rotor wheel 5 at predetermined intervals in the circumferential direction of the rotor wheel 5. The tip of the moving blades 6 are attached to a cover 7 so that leakage of working fluid is prevented. Both the nozzle blades 4 and the moving blades 6 form a working fluid passage of this stage of the turbine.
A next (second) stage of the turbine, which is located at a downstream side of the above (first) stage, has a rapidly enlarged passage for the working fluid. This passage is composed of a nozzle blade passage and a moving blade passage as well as the above working fluid passage. The nozzle blade passage is formed by an outer diaphragm ring 8, an inner diaphragm ring 9 and nozzle blades 10. The moving blade passage is formed by both moving blades 12 implanted in a rotor wheel 11 and a cover 13 attached to the tip of the moving blades 12.
In the second stage, the working fluid expands from a high-pressure condition to a low-pressure condition through the passage, so the specific capacity (volume) of the fluid enlarges. To correspond to such enlargement of specific capacity, the inner wall of the passage is inclined in such a manner that the area of the passage is enlarged in the downstream direction.
Through the above-mentioned passage of the two stages, the working fluid generates a secondary flow at the nozzle blades 4 and 10. This mechanism of generating the secondary flow will now be described with reference to FIG. 20.
When the working fluid, which is high-pressure steam or the like, flows in the nozzle blade passage between the nozzle blades, the working fluid is curved into a circular arc form in the nozzle blade passage as indicated with a two-dot chain line shown in FIG. 20. At this time, centrifugal components are generated in a direction from an extrados E of the nozzle blade 4 to an intrados F of an adjacent nozzle blade 4. Since the centrifugal components and the pressure in the nozzle blade passage are in equilibrium, the static pressure at the intrados F of the nozzle blade 4 is raised.
On the other hand, the pressure at the extrados E of the nozzle blade 4 is lowered because the flow velocity of the working fluid is high along the extrados E. As a result, a pressure gradient is generated in a region of the nozzle blade passage from the intrados F of the nozzle blade 4 to the extrados E of an adjacent nozzle blade 4. As shown in FIG. 20, also a pressure gradient of the foregoing type is generated between the inner wall of the root of the nozzle blades and a layer adjacent to the outer surface of the tip of the nozzle blades in which the flow velocity is low, that is, in the boundary layer. In the portions adjacent to the boundary layer, the flow velocity is low and the acting centrifugal component is weak. Therefore, the flow of the working fluid cannot withstand the pressure gradient generated in a direction from the intrados F of the nozzle blade 4 to the extrados E of an adjacent blade. As a result, the flows are generated in a direction from the intrados F of the nozzle blade 4 to the extrados E of an adjacent nozzle blade, as indicated with symbols f1 and f2 shown in FIG. 20. The flows f1 and f2 collide with the extrados E of the nozzle blade 4 and curl up. As a result, secondary flow eddies 14a and 14b are generated adjacent to the inner wall of the root of the nozzle blades 4 and the outer wall of the tip of the same.
FIG. 21 is a diagram showing a mechanism of the moving blades 6 disposed downstream from the nozzle blades 4 to generate a secondary flow. Since the mechanism of the secondary flow the moving blades 6 is substantially the same as the mechanism of the nozzle blades 4 to generate eddies in the secondary flow. Features that are similar to those shown in FIG. 20 are given the same reference numerals and symbols. As can be understood from FIGS. 22 and 23 showing losses of the nozzle blades 4 and the moving blades 6, eddy losses are caused from the secondary flow eddies. Thus, excessive losses are produced in the portions adjacent to the inner and outer walls of the turbine blades.
If secondary flow eddies 14a and 14b are generated, a portion of energy of the working fluid is dispersed. Moreover, non-uniform flows of the working fluid are formed, thus causing a problem to arise in that losses of the nozzle blades and the moving blades are enlarged and the performance of the stages deteriorate excessively.
To prevent the secondary flow loss caused by the secondary flow eddies 14a and 14b generated in the abovementioned passage (stages), a variety of techniques have been researched and developed. For example, a nozzle blade having a reduced outer surface has been employed. This reduced outer surface has irregularities formed in the tip of the nozzle blade to reduce the height of the flow passage in the downstream direction. FIG. 24 is a cross sectional view showing a turbine nozzle having the nozzle blade 15 having a reduced outer surface. The nozzle blade 15 having the reduced outer surface causes flows along the outer surface of the nozzle blade 15. Thus, the flow line is shifted toward the inside portion (toward the central portion) of the nozzle blade passage as indicated with an arrow ft. This configuration further provides that the flow lines in the central portion and the root (inside) portion are shifted inwards (toward the central portion), as indicated by arrows fp and fr, in a manner similar to those along the outer surface. As a result, the flow lines push the flows to the inner wall of the nozzle blade 15 in portions adjacent to the root of the nozzle blade 15. Thus, enlargement of the boundary along the inner wall can be prevented so that enlargement of the loss caused by the secondary flow eddies is prevented.
FIG. 25 shows a distribution of reduced losses attributable to the effect of the conventional nozzle blade 15 having the reduced outer surface to prevent enlargement of losses caused by eddies in the secondary flow. As can be understood from FIG. 25, losses can be significantly reduced in the portions adjacent to the root of the nozzle blade. Improvement in the performance has been confirmed also in overall efficiency experiments of the turbine stages.
The fact that the nozzle blade 15 having the reduced outer surface is able to improve the performance has been confirmed in the above-mentioned stage efficiency experiments. However, local separation of the flow at the tip of the nozzle blade takes place that is attributable to a rapid shift of the flow line, as shown in the distribution of losses in the trailing edge of the nozzle blade. Therefore, the secondary flow cannot satisfactorily be improved.
Moreover, a large portion of the working fluid flows adjacent to the root of the nozzle blade. Therefore, considerable change in the flow rate occurs in the direction of the height of the nozzle blade.
Therefore, the stage performance realized by the nozzle blade 15 having the reduced outer surface can be further improved. That is, a nozzle blade passage is required which is capable of preventing separation of flows of the working fluid and improving the flow rate characteristic at the tip of the nozzle blade.